Double Factorial Calculator

Double factorial is represented by "m!!". When m is a natural number, it represents the product of all positive integers not exceeding m and having the same parity as m.

Example:
3!!=1*3=3
5!!=1*3*5=15
6!!=2*4*6=48
8!!=2*4*6*8=384
In addition, 0!!=1!!=1